Substantially parallel light rays proceeding from an object viewed at a distance and entering the emmetropic or optically normal eye are brought to focus on the retina. When the object viewed is six meters or more from the eye, the ciliary muscle is relaxed and light rays entering the eye from distant objects are focused on the retina. When such relaxation is maintained, diverging light rays from objects closer to the observer would establish their focus behind the retina. Light rays proceeding from objects closer than six meters are brought to focus on the retina as a result of an increase in the curvature or refractive power of the crystalline lens in the human eye. This physiologic process by which the form and density of the crystalline lens is changed is called accommodation, and ideally results in clear central vision over a full range of distances. The failing or absence of accommodation resulting in presbyopia and the lack of accommodation in an aphakic eye may be addressed in the design of the contact lens or intraocular lens which corrects the refractive error and in some manner and to some degree the accommodative insufficiency or absence of accommodation in the presbyopic or aphakic patient. Numerous contact lens and intraocular lens designs have been proposed to provide an accommodative effect similar in various respects to that of physiologic accommodation.
In U.S. Pat. No. 4,580,882, a continuously variable contact lens is described incorporating a concave aspheric surface of revolution and which has continuously varying refractive power extending from the center region of the lens peripheralward. In this lens design, the refractive power to achieve the desired distance vision correction is located in the center region of the lens with the power increasing peripheralward, up to 9.7 mm in diameter, to the desired near vision power. A problem exists in that such a lens design assumes a maximum pupillary aperture to achieve the desired refractive power for both distance and near vision requirements. It should be evident that such an assumption is not valid under many circumstances and the benefits of such a lens design will degrade accordingly, dependent upon actual conditions encountered by the patient.
In U.S. Pat. No. 4,418,991, a presbyopic contact lens is described as having a spherical anterior surface and a posterior surface of revolution with an annular fitting region as well as an optical region having a central portion providing the distance correction, with a paracentral portion providing a gradient of diopter adds for close and intermediate viewing. In the paracentral portion, the increasing radii of curvature away from the center results in a gradient of diopter add achieved by a continuous flattening of the paracentral area away from the center to between 4 mm and 51/2 mm. As stated in this patent, the pupil of the eye has a diameter of about 4 mm to 5 mm in an indoor situation, but may be significantly smaller in conditions where there is a greater amount of light. In normal daylight conditions, the iris aperture may be reduced to from about 2 mm to 3 mm. Again, it should be evident that although the optical area of the lens in this prior invention is designed in an attempt to provide multifocal capabilities, limitations exist with respect to the pupillary aperture which will physically vary depending upon the conditions the wearer encounters, thereby effecting the focusing qualities of the lens.
Similarly, intraocular lenses are implanted in the eye as a replacement of the absent human crystalline lens. In U.S. Pat. No. 4,710,193, an aspheric posterior chamber intraocular lens is described which has at least one convex aspheric surface of revolution designed to provide continuously and regularly increasing refractive power from its apex peripheralward in its optically active area. This invention is stated to correct the axial refractive error of the aphakic eye and to produce clear central vision over a continuous range of distances from near to far. Similar problems exist with regard to the desired refractive correction characteristics of this intraocular lens design with respect to changes in the size of pupillary aperture.